The present invention relates to cascade cooling systems capable of continuous operation despite the failure of certain components therein. The system also prolongs the life and improves the reliability of compressors used therein by operating them intermittently.
In the prior art relating to low temperature continuously operated three stage cascade cooling apparatus numerous applications can be found in storage freezers, environmental test chambers, metal treating baths, etc., where each cooling stage is comprised of a single refrigerating motor compressor. In most cases at the medium and low temperature stage cooling level, the motor compressor operation is for a short period of time. Continuous operation for periods of months and years has not been completely satisfactory with single motor compressors particularly when operated in high ambient environment and with fluorinated refrigerants as the cooling medium. It has been difficult to provide long term effective motor compressor lubrication because fluorinated refrigerants tend to displace and wash the oil film from reciprocating motor compressor components. This is due to low oil solubility and the difficulty in keeping the oil in the motor compressor. A further handicap to overcome in a conventionally designed three stage low temperature cascade cooling apparatus operated with single motor compressors for each stage is the necessary periodic termination of the cooling apparatus in order to limit friction increase in the motor compressor reciprocating components. Increased friction will lead to motor compressor seizure and electric winding burn out. A further handicap to overcome is that during failure of a single motor compressor in a three stage cascade cooling apparatus arrangement, the process of refrigerating the storage enclosure stops and rapid temperature rise occurs. Unless a back up CO2 or LN2 liquid injection cooling system is provided, the storage enclosure contents have to be transferred to a standby enclosure immediately to prevent their spoilage.